Performance of a communication system comprising a transmitter, a receiver, and a communication channel, is highly sensitive to the analog-to-digital converter (ADC) sampling phase when ADC operates at symbol rate. Performance of the communication system is defined as signal-to-noise (SNR) ratio at the output of the decision device of the receiver. To compensate for that sensitivity and because of low enhancement of noise, a Decision Feedback Equalizer (DFE) is preferred over a symbol rate Linear Equalizer (LE) at the receiver because a DFE may have an adaptive feedback mechanism capable of better correcting the corrupted received data and variations in the channel compared to the LE based receiver. A THP communication system is commonly derived from the DFE system by moving the feedback part of the DFE to the transmitter. Some key advantages of the DFE based systems are lost in the THP communications system, since THP system has a linear receiver, without an adaptive feedback mechanism.
Referring back to the DFE based system, since the estimated data at the receiver can be erroneous because, for example, the noise coupled with the received signal and intersymbol interference, the feedback mechanism of the DFE receiver will further generate an incorrect data. In other words, erroneous data will enter into the feedback mechanism of the DFE based receiver which will generate inaccurate feedback filter output. This inaccurate output is subtracted from the received signal. The subtracted signal is then input to a decision device that further generates inaccurate data. The inaccurate data from the decision device then enters the feedback filter again, and so the cyclic process continues to generate inaccurate data. This cyclic process is known as error propagation.
To fix this problem of error propagation, a THP communication scheme is employed. Unlike the DFE based system, the THP system comprises a feedback filter that resides in the transmitter and hence actual transmit data and not estimated data (which may be erroneous) enters the feedback filter. Since the data for transmission is the actual data rather than estimated data, there is no problem of error propagation in THP based communication system.
However, the transmitter in the THP system is not capable to cope with error associated with the sampling phase of the ADC at the receiver because the receiver is in LE mode i.e. there is no feedback mechanism at the receiver. Indeed, the filter coefficients at the transmitter of the THP system are derived for a particular sampling phase of the ADC and are frozen once derived. This means that should sampling phase of the ADC change there is no adaptive feedback mechanism in the THP communication system to adjust the system to the new sampling phase of the ADC, and so the system relies on adaptive mechanism of only the Linear Equalizer at the receiver. Hence, the receiver may still experience a low SNR (thus lower performance) at the output of its decision device if the sampling phase of the ADC changes.